1. Field of the Invention
The present invention relates to a novel production method of 4,6-diaminoresorcin which is a monomer for polybenzobisoxazole (PBO). More particularly, it relates to a production method of 4,6-diaminoresorcin in which resorcin is used as a starting material, any step of forming a halogen-containing compound is not required and the production of by-products is inhibited. Furthermore, the present invention relates also to a novel intermediate useful for the synthesis of 4,6-diaminoresorcin and a production method of this intermediate.
A PBO fiber is superior to conventional fibers in terms of various properties such as strength, modulus of elasticity, heat resistance and chemical resistance, as disclosed in Japanese Patent Publication No. 501452/1986, so that it is expected to apply this kind of fiber to various uses including structural materials and thermal insulating materials as a super fiber which is superior to aramid, and hence the fiber is considered to be an industrially extremely useful resin. 4,6-Diaminoresorcin is a monomer for the PBO, and therefore, it is important as a raw material for the PBO.
2. Description of the Related Art
A PBO is a polymer represented by the following general structural formula (c) and can be obtained through a condensation reaction between a compound (a) and a compound (b) as represented by the following reaction formula: 
wherein Ar is an aromatic group; and Y is a functional group having an electron-deficient carbon such as a carboxyl group, a carboxylic acid halide group, a haloalkyl group or a nitrile group.
The thus obtained polybenzobisoxazole is used as fibers, films and the like, but its physical properties such as strength and modulus of elasticity are greatly influenced by a polymerization degree of the polymer. It is known that in a polycondensation reaction, a maximum polymer viscosity is generally obtained when a feed ratio of the monomers is 1:1, and the polymer viscosity rapidly decreases as the feed ratio of the monomers deviates from a ratio of 1:1. That is, to attain a satisfactory sufficient polymerization degree, it is necessary to strictly control the feed ratio of the monomers.
However, in the case that the monomers contain impurities in large quantities, it is difficult to control this feed ratio of the monomers. Particularly, when the monomers contain even small amounts of monoamine and triamines which act as polymerization terminators, the deterioration of the polymerization degree is caused. Therefore, there have been desired the high-purity monomers containing neither the monoamine nor the triamines.
As a monomer (a) for the PBO, 4,6-diaminoresorcin is known, and several synthesis methods for this compound have been reported.
A conventional production method of 4,6-diaminoresorcin comprises synthesizing dinitroresorcin as a precursor by a method in which resorcin is acetylated and then nitrated (Ber. Dtsch. Chem. Ges., 16, 552, 1883), a method in which 1,3-bis(alkyl carbonate)benzene is nitrated (Japanese Patent Application Laid-Open No. 136/1990) or another method, and then reducing the thus synthesized dinitroresorcin.
However, the operation of this conventional method is complicated and a manufacturing cost increases, because protecting groups are introduced for the hydroxy groups of resorcin, and from an industrial viewpoint, the above method has a problem that the protecting groups eliminated in a hydrolysis step become an unrecoverable by-product, and a problem that a trinitro compound is produced in the nitration step and a triamino compound is produced in the reduction step, and they disturb the polymerization in the synthesis of the PBO.
Furthermore, several methods using no protecting groups have been proposed in which a halobenzene is used as a raw material, and there are known, for example, a method in which a trichlorobenzene is nitrated (Japanese Patent Application Laid-Open No. 500743/1990) and a method in which a dihalobenzene is nitrated and then hydrolyzed with an alkali (Japanese Patent Application Laid-Open Nos. 238561/1989, 233127/1995, 316102/1995 and 73417/1996).
In these methods, however, since 4,6-dinitroresorcin is unstable under the alkali conditions in the hydrolysis step the operation of these methods is apt to be complicated in order to avoid the decomposition of produced 4,6-dinitroresorcin. Furthermore, trichlorobenzene and its nitrated compound have a problem that they are strongly poisonous and cause an irritation on skin. Therefore, it is not preferable in consideration of the safety of an operator to pass through the production of a halogen-containing compound such as the halobenzene and its nitrated compound.
In addition, another method which comprises subjecting an aniline to diazotization and diazo-coupling the thus diazotized compound to resorcin, followed by hydrocracking is disclosed in Japanese Patent Application Laid-Open Nos. 242604/1995 and 124575/1997. In this method, however, aniline produced by the hydrocracking might be mixed with the product, and might disturb the polymerization in the synthesis of the PBO.
It is an object of the present invention to provide a novel production method of 4,6-diaminoresorcin in which any step of forming a halogen-containing compound is not required and the production of by-products is inhibited.
It is another object of the present invention to provide a production method of 4,6-dinitroresorcin, as a precursor of 4,6-diaminoresorcin, via the production of a novel intermediate.
It is still another object of the present invention to provide a production method of a high-molecular-weight PBO by the use of high-purity 4,6-diaminoresorcin obtained by these methods.
The present inventors have made intensive studies to solve the above problems, and found that 4,6-diaminoresorcin can be obtained at a high yield by sulfonating resorcin to form resorcin 2,4,6-trisulfonate, nitrating resorcin 2,4,6-trisulfonate to obtain 2-sulfonic acid-4,6-dinitroresorcin with a high position selectivity, hydrolyzing this compound to form 4,6-dinitroresorcin, and then reducing the same. In consequence, the present invention has been attained.
Furthermore, the present inventors have found that a high-molecular-weight PBO can be obtained by hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin to obtain 4,6-dinitroresorcin containing neither isomers nor trinitro compounds, reducing this compound to obtain high-purity 4,6-diaminoresorcin, and then polymerizing the same. Thus, the present invention has been achieved.
That is, the present invention includes the following aspects.
1. A production method of resorcin 2,4,6-trisulfonate which comprises the step of bringing resorcin into contact with a sulfonating agent.
2. The production method of resorcin 2,4,6-trisulfonate according to the above (1), wherein fuming sulfuric acid is used as the sulfonating agent.
3. The production method of resorcin 2,4,6-trisulfonate according to the above (2), wherein fuming sulfuric acid to be used contains 3 mols or more of free SO3 per mol of resorcin.
4. A production method of 2-sulfonic acid-4,6-dinitroresorcin which comprises the step of nitrating resorcin 2,4,6-trisulfonate.
5. The production method of 2-sulfonic acid-4,6-dinitroresorcin according to the above (4), wherein the nitration is carried out in sulfuric acid or a fuming sulfuric acid solvent.
6. A production method of 2-sulfonic acid-4,6-dinitroresorcin which comprises the following steps:
(1) a first step of producing resorcin 2,4,6-trisulfonate by bringing resorcin into contact with a sulfonating agent, and
(2) a second step of producing 2-sulfonic acid-4,6-dinitroresorcin by bringing resorcin 2,4,6-trisulfonate into contact with a nitrating agent.
7. A production method of 4,6-dinitroresorcin which comprises the step of hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin.
8. The production method of 4,6-dinitroresorcin according to the above (7), wherein the hydrolysis is carried out in water or an aqueous mineral acid solution.
9. The production method of 4,6-dinitroresorcin according to the above (8), wherein sulfuric acid is used as the mineral acid.
10. A production method of 4,6-dinitroresorcin which comprises the following steps:
(1) a first step of producing resorcin 2,4,6-trisulfonate by bringing resorcin into contact with a sulfonating agent,
(2) a second step of producing 2-sulfonic acid-4,6-dinitroresorcin by bringing resorcin 2,4,6-trisulfonate into contact with a nitrating agent, and
(3) a third step of producing 4,6-dinitroresorcin by hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin.
11. A production method of 4,6-diaminoresorcin which comprises the following steps:
(1) a first step of producing resorcin 2,4,6-trisulfonate by bringing resorcin into contact with a sulfonating agent,
(2) a second step of producing 2-sulfonic acid-4,6-dinitroresorcin by bringing resorcin 2,4,6-trisulfonate into contact with a nitrating agent,
(3) a third step of producing 4,6-dinitroresorcin by hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin, and
(4) a fourth step of producing 4,6-diaminoresorcin by reducing 4,6-dinitroresorcin.
12. A production method of polybenzobisoxazole which comprises the steps of hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin, followed by reducing to obtain 4,6-diaminoresorcin, and then reacting the thus obtained-4,6-diaminoresorcin with aromatic dicarboxylic acid. 13. 2-Sulfonic acid-4,6-dinitroresorcin represented by the following formula and salts thereof: 
wherein M is hydrogen, an alkali metal or an alkaline earth metal, and n is 1 or 2.
14. A production method of 4,6-diaminoresorcin which comprises:
(1) a first step of producing 4,6-dinitroresorcin by hydrolyzing 2-sulfonic acid-4,6-dinitroresorcin, and
(2) a second step of producing 4,6-diaminoresorcin by reducing 4,6-dinitroresorcin.
15. The production method of 4,6-diaminoresorcin according to the above (14), wherein 2-sulfonic acid-4,6-dinitroresorcin is obtained by the following steps:
(1) a first step of producing resorcin 2,4,6-trisulfonate by bringing resorcin into contact with a sulfonating agent, and
(2) a second step of producing 2-sulfonic acid-4,6-dinitroresorcin by bringing resorcin 2,4,6-trisulfonate into contact with a nitrating agent.
16. The production method of 4,6-diaminoresorcin according to the above (14), wherein in the second step, 4,6-dinitroresorcin is reduced in an aqueous mineral acid solution.
17. The production method of 4,6-diaminoresorcin according to the above (16), wherein hydrochloric acid is used as the mineral acid.
18. The production method of 4,6-diaminoresorcin according to the above (14) which comprises the steps of dissolving or suspending 4,6-dinitroresorcin in a solvent, adjusting the pH of the suspension in a range of 4 to 5 to obtain 4,6-dinitroresorcin, and then reducing the thus obtained 4,6-dinitroresorcin.
A production method of 4,6-diaminoresorcin of the present invention can be accomplished via the following intermediate compounds. 
First, resorcin (i.e., 1,3-benzenediol) (A) as a raw material is brought into contact with a sulfonating agent (reaction R1) to obtain resorcin 2,4,6-trisulfonate (B). Successively, sulfonic groups at positions 4 and 6 are selectively nitrated (reaction R2) to obtain 2-sulfonic acid-4,6-dinitroresorcin (C), which is then hydrolyzed (reaction R3) to obtain 4,6-dinitroresorcin (D). Finally, 4,6-dinitroresorcin (D) is reduced (reaction R4) to obtain desired 4,6-diaminoresorcin (E).
To produce resorcin 2,4,6-trisulfonate, as described in Berichite, 10, 182, there is known a method in which a disulfonic acid resorcin is heated at 200xc2x0 C. in fuming sulfuric acid. However, in the case that the thus formed disulfonic acid resorcin is isolated and then used in the subsequent step, a process becomes complicated and the serious decrease of yield occurs. Thus, the above method has been unsuitable for industrial practice.
Next, each of the reaction steps R1 to R4 will be described in detail hereinafter.
In the first reaction step R1, a sulfonating agent can be used that sulfonates resorcin (A) to produce resorcin 2,4,6-trisulfonate (B). Illustrative examples of the sulfonating agent include concentrated sulfuric acid, fuming sulfuric acid and sulfur trioxide. The reaction may be carried out using an appropriate solvent. However, it is industrially advantageous that the reaction is carried out in excess concentrated sulfuric acid or fuming sulfuric acid without using a solvent. To avoid desulfonation caused by hydrolysis, concentrated sulfuric acid or fuming sulfuric acid is preferably used at a concentration of 80 to 100% by weight, more preferably 95% by weight or more. Above all, the use of fuming sulfuric acid is most preferable.
According to the investigation of the present inventors, the selectivity of resorcin 2,4,6-trisulfonate depends largely on the concentration of SO3 in sulfuric acid. As the SO3 concentration in sulfuric acid decreases, the selectivity of resorcin 2,4,6-trisulfonate decreases. For example, even if 95 wt % sulfuric acid (SO3 concentration=77.6%) is used, the preparation of resorcin 2,4,6-trisulfonate is possible, but it has been confirmed that in the case that 95 wt % sulfuric acid (SO3 concentration=77.6%) is used, the selectivity of resorcin 2,4,6-trisulfonate is in a range of about 12 to 17 mol %, and remaining 83 to 88 mol % thereof is resorcin 4,6-disulfonate. This fact can be considered to be due to the decrease of the sulfuric acid concentration by water produced during the reaction. To attain such a yield as can be industrially satisfied, it is desirable that the SO3 concentration in sulfuric acid at the completion of the reaction is 81.6% or more, i.e., the sulfuric acid concentration is about 100% or sulfuric acid contains an excessive amount of free SO3. To maintain such a state, the sulfonation should be carried out by using fuming sulfuric acid containing 3 mols or more of free SO3 per mol of resorcin.
The amount of the sulfonating agent to be used is not particularly limited, as long as it satisfies the above SO3 concentration. It is, however, preferably 5 to 50 times more by weight than resorcin in view of volumetric efficiency and efficient agitation.
To bring resorcin into contact with the sulfonating agent, one of them may be added to the other or vice versa.
As for a reaction temperature, the reaction may be carried out within any temperature range in which the desired product can be obtained. However, a temperature range of from about 0 to about 200xc2x0 C. is preferable. To avoid the installation of large cooling facilities which is required for heat generation at the time of the sulfonation, the reaction temperature is desirably 20xc2x0 C. or higher. To prevent an undesirable side reaction, the reaction temperature is desirably 150xc2x0 C. or lower.
Resorcin 2,4,6-trisulfonate can be isolated from a reaction mass by adding dropwise the reaction mass to an aqueous solution of an inorganic salt such as sodium sulfate to cause salting-out, followed by filtering and drying.
Next, reference will be made to the reaction step R2 in which 2-sulfonic acid-4,6-dinitroresorcin (C) is obtained from resorcin 2,4,6-trisulfonate (B). The reaction step R2 is a nitration step, in which a known nitrating agent capable of producing the desired compound can be used. Illustrative examples of the nitrating agent include nitric acid, fuming nitric acid, and nitrates such as sodium nitrate and potassium nitrate. The nitration may be carried out after the isolation of resorcin 2,4,6-trisulfonate from the sulfonated reaction mass as described above, but it is industrially advantageous that the nitration is carried out in a one-pot manner by adding the nitrating agent to the sulfonated reaction mass (in this case, the nitration is carried out in sulfuric acid or a fuming sulfuric acid solvent). The amount of the nitrating agent to be used is in a range of about 1 to 10 mols per mol of resorcin (A) as a starting material, and in order to sufficiently promote the reaction and to inhibit the excessive nitration, it is preferably in a range of about 2 to 4 mols per mol of resorcin (A).
The reaction step R2 can also be carried out within any temperature range in which the desired product can be obtained. However, when the reaction temperature is too high, the reaction proceeds quickly, whereby the undesirable side reaction may occur on occasion. Therefore, the reaction is usually carried out while the reaction temperature is controlled by cooling. The reaction temperature is preferably in a range of about 0 to 80xc2x0 C., more preferably about 0 to 50xc2x0 C.
To isolate desired 2-sulfonic acid-4,6-dinitroresorcin (C) from the reaction mass after the completion of the reaction, the reaction mass is first neutralized with an alkali to form an alkali metal salt or an alkaline earth metal salt, which is then subjected to salting-out to thereby deposit the salt, so that the desired salt is obtained as a mixture with an alkali metal salt or an alkaline earth metal salt of sulfuric acid. In succession, the mixture is added to, for example, a mixed solution containing water and ethanol in a ratio of 2:8, and the resultant solution is then heated at 50 to 80xc2x0 C. to dissolve the desired compound. Afterward, the inorganic salt is removed by filtration under heating. The resultant filtrate is cooled to deposit yellow crystals, which are then filtered to thereby obtain an alkali metal salt or an alkaline earth metal salt of 2-sulfonic acid-4,6-dinitroresorcin (C). Alternatively, the solvent may be removed from the filtrate to obtain the alkali metal salt or the alkaline earth metal salt of 2-sulfonic acid-4,6-dinitroresorcin (C). The alkali metal salt or the alkaline earth metal salt can be desalted by dissolving the salt in water, passing the solution through a column filled with a strongly acidic cation exchange resin, and removing water therefrom. In consequence, 2-sulfonic acid-4,6-dinitroresorcin (C) is obtained.
Next, reference will be made to the reaction step R3 in which 2-sulfonic acid-4,6-dinitroresorcin (C) is hydrolyzed to obtain 4,6-dinitroresorcin (D).
The hydrolysis is carried out in water or an aqueous solution containing an acid or an alkali as a catalyst. However, it is preferably carried out in water or the acid-containing aqueous solution, more preferably in a mineral acid-containing aqueous solution, because there is a fear that 4,6-dinitroresorcin (D) after the hydrolysis may bring about a further decomposition reaction in the alkali aqueous solution having a high concentration. Examples of the mineral acid which can be used herein include sulfuric acid, hydrochloric acid and phosphoric acid. The mineral acid is preferably sufficiently diluted with water, or a sulfuric acid coupling agent may be added thereto in order to avoid the recombination of the separated sulfonic group. In the hydrolysis step R3, the concentration of the mineral acid is preferably in a range of 5 to 90% by weight, and in order to maintain a sufficient hydrolysis rate and to obtain a sufficient yield, it is desirably 10% by weight or more. The amount of the aqueous mineral acid solution to be used is not particularly limited, but it is preferably about 2 to 50 times by weight more than 2-sulfonic acid-4,6-dinitroresorcin (C) in view of agitation efficiency and volumetric efficiency. The reaction temperature is preferably in a range of from 50xc2x0 C. to reflux temperature or so.
This hydrolysis step R3 may be carried out after 2-sulfonic acid-4,6-dinitroresorcin (C) has been isolated, or alternatively it may be carried out without isolating 2-sulfonic acid-4,6-dinitroresorcin (C) from the reaction mass in the nitration step R2.
When the hydrolysis is directly carried out without isolating 2-sulfonic acid-4,6-dinitroresorcin (C), the reaction mass in the nitration step R2 is diluted so as to become the aqueous mineral acid solution which meets predetermined conditions. Furthermore, when 2-sulfonic acid-4,6-dinitroresorcin is isolated as the alkali metal salt or the alkaline earth metal salt, the salt may be directly used as it is in the hydrolysis step. In addition, the salt may be used in the form of a mixture with an alkali sulfate.
As the hydrolysis reaction proceeds, crystals of 4,6-dinitroresorcin (D) are gradually deposited. Therefore, these crystals are filtered after the completion of the reaction to obtain the desired product. The thus obtained 4,6-dinitroresorcin (D) may be purified as required prior to its use. The compound 4,6-dinitroresorcin (D) may be purified by subjecting it to sludging in or recrystallization from a solvent such as ethanol. However, it is preferable for the purpose of preventing the deterioration of a catalytic activity in the reduction step to dissolve or suspend 4,6-dinitroresorcin (D) in a solvent such that the resulting solution has a pH of 4 to 5. Concretely, any of the following procedures can be taken.
(1) An alkali is added to a solution or slurry in which 4,6-dinitroresorcin is dissolved or suspended in a solvent, thereby adjusting the pH in a range of 4 to 5.
(2) 4,6-Dinitroresorcin is dissolved in a two-layer mixed solvent of water and a hydrophobic solvent, and an alkali is then added to the solution, thereby adjusting the pH in a range of 4 to 5.
(3) 4,6-Dinitroresorcin, which is in the form of an alkali salt, is dissolved in water, and an acid is then added to the solution, thereby adjusting the pH in a range of 4 to 5.
Examples of a hydrophilic solvent which is one type of the solvent to be used include water, methanol, ethanol, n-propanol, iso-propanol, DMI (1,3-dimethyl-2-imidazolidinone) and DMF (N,N-dimethylformamide). Examples of a hydrophobic solvent which is the other type of the solvent to be used include ethyl acetate, 1,3-dimethoxybenzene, phenetole and anisole. The amount of the solvent depends on the kind of solvent to be used, but it is in a range of 1 to 100 times more than that of 4,6-dinitroresorcin. The above alkali is not particularly limited, but its examples include potassium hydroxide, sodium hydroxide, potassium carbonate and sodium carbonate. The above acid is not particularly limited, but its examples include mineral acid such as hydrochloric acid and sulfuric acid. A temperature at the pH adjustment depends on the kind of solvent to be used, but it is preferably in a range of 10 to 80xc2x0 C.
The isolation of purified 4,6-dinitroresorcin, in the case that the pH adjustment is made in the slurry state, is carried out by filtrating the slurry as it is, washing the collected substance with the used solvent, and further washing it with water. In the case that the pH adjustment is made in the two-layer heterogeneous state, the isolation is carried out by allowing the obtained solution to stand for separation, washing the separated organic phase with water, cooling it for crystallization, and then filtering the resultant crystals. In the case that 4,6-Dinitroresorcin in the form of the alkali salt is dissolved and the acid is then added, the isolation is carried merely by filtrating the obtained mixture as it is, and then washing it with water.
Finally, reference will be made to the reduction step R4 in which 4,6-diaminoresorcin (E) is obtained from 4,6-dinitroresorcin (D).
In this step, any reduction technique may be used as long as the desired product is obtained. However, a catalytic reduction is usually carried out in the presence of a noble metal catalyst. The noble metal catalyst to be used herein is a platinum group metal such as palladium, platinum, rhodium or ruthenium which is carried on a proper carrier, and it is preferable to use palladium or platinum carried on carbon.
The amount of the catalyst to be used is in a range of 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on 4,6-dinitroresorcin (D). A reaction temperature is in a range of 20 to 100xc2x0 C., and a hydrogen pressure is in a range of 0.1 to 10 MPa.
The solvent usable in the reaction is water, an organic solvent, an organic acid, or a mixture of water and a mineral acid. Examples of the usable mineral acid include hydrochloric acid, phosphoric acid and sulfuric acid. Above all, the employment of hydrochloric acid is preferable, because a hydrochloride of 4,6-diaminoresorcin is formed simultaneously with the reduction of 4,6-dinitroresorcin and this hydrochloride is dissolved in water to form a homogeneous solution which is easy to handle. Examples of the organic solvent include aromatic hydrocarbons such as benzene and toluene, and alcohols such as methanol and ethanol. Examples of the organic acid include acetic acid and propionic acid.
The formed 4,6-diaminoresorcin (E) is converted into a mineral acid salt to avoid oxidation/decomposition, and this salt is then isolated by a known technique such as precipitation or filtration. More specifically, for example, the reaction mass is added to a diluted aqueous hydrochloric acid solution containing stannous chloride to dissolve 4,6-diaminoresorcin hydrochloride. In succession, the resulting solution is filtered to remove the catalyst, and then the solvent is distilled off under reduced pressure. Alternatively, the filtrated solution is mixed with concentrated hydrochloric acid to deposit crystals, followed by filtration. When the aqueous hydrochloric acid solution is used as the solvent, 4,6-diaminoresorcin hydrochloride is already formed in the reduction reaction mass, and hence the steps of the salt-forming and the dissolving are not necessary. When phosphoric acid or sulfuric acid is used as the mineral acid and the 4,6-diaminoresorcin mineral acid salt is deposited, the salt is dissolved in the form of 4,6-diaminoresorcin hydrochloride by salt-exchanging, and crystals are then deposited in the similar manner as above.
An obtained 4,6-diaminoresorcin dimineral acid salt can be further purified by a technique such as recrystallization. Concretely, for example, after crude 4,6-diaminoresorcin dimineral acid salt is dissolved in water including stannous chloride, activated carbon is added thereto, followed by treatment. Next, the activated carbon is removed by filtration, and concentrated hydrochloric acid is then added thereto for crystallization.
In order to obtain a PBO by the use of the thus obtained 4,6-diaminoresorcin (E), a known polymerization method can be employed. For example, the PBO can be obtained by dissolving the 4,6-diaminoresorcin dimineral acid salt in polyphosphoric acid, heating the resulting solution under reduced pressure to remove hydrochloric acid, adding a required amount of diphosphorus pentoxide, further adding aromatic dicarboxylic acid in a nearly equimolar amount to 4,6-diaminoresorcin, and then stirring the mixture under heat. Examples of the usable aromatic dicarboxylic acid include terephthalic acid, isophthalic acid, 4,4xe2x80x2-bis(benzoic acid), 4,4xe2x80x2-oxybis(benzoic acid) and 2,6-naphthalene dicarboxylic acid.